Conventionally, there is the opening device formed in the lid of the thermoplastic resin injection molded to the top part of the paper container filled up with the liquid food, such as the milk and soft drinks. In the opening device, as for the packaging material of the liquid food container, paper substrate is covered by an outer layer and an inner layer of thermoplastic resin, a straw hole is covered with thermoplastic resin and a straw hole sealing part is formed. The straw hole sealing part is formed by injection molding. In the straw hole sealing part, when piercing with the straw, the straw does not slip to easily penetrate it, and the straw hole sealing part is necessarily soft not to bend the straw tip part. Therefore, a thin-walled part is formed in the straw hole sealing part by the extent that sealing performance is not damaged.
Another opening device has a lid of thermoplastic resin injection molded to a top part of the paper container filled up with the liquid food, and a thin-walled part for straw holes formed in the lid.
On the thin-walled part, the sealing of the straw hole can be penetrated with the straw. However, there is a risk that the sealing of the thin-walled part may break by the vibration etc. in the conveyance/ distribution process of the liquid food container. It is very difficult to control the balance of sealing and unsealing by means of the thin-walled part of the straw hole.